dicobalt phosphide microspheres as Mott–Schottky electrocatalyst for efficient and stable hydrogen evolution reaction in wide-pH environment

Journal Pre-proofs Metal-Organic Frameworks Derived Carbon-Incorporated Cobalt/Dicobalt Phosphide Microspheres as Mott–Schottky Electrocatalyst for Efficient and Stable Hydrogen Evolution Reaction in Wide-pH Environment Huangze Yu, Junfeng Li, Guoliang Gao, Guang Zhu, Xianghui Wang, Ting Lu, Likun Pan PII: DOI: Reference:

S0021-9797(20)30076-X https://doi.org/10.1016/j.jcis.2020.01.059 YJCIS 25932

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

26 November 2019 15 January 2020 16 January 2020

Please cite this article as: H. Yu, J. Li, G. Gao, G. Zhu, X. Wang, T. Lu, L. Pan, Metal-Organic Frameworks Derived Carbon-Incorporated Cobalt/Dicobalt Phosphide Microspheres as Mott–Schottky Electrocatalyst for Efficient and Stable Hydrogen Evolution Reaction in Wide-pH Environment, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.059

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Metal-Organic Frameworks Derived Carbon-Incorporated Cobalt/Dicobalt Phosphide Microspheres as Mott–Schottky Electrocatalyst for Efficient and Stable Hydrogen Evolution Reaction in Wide-pH Environment

Huangze Yua, Junfeng Lia, Guoliang Gaoa, Guang Zhub, Xianghui Wanga*, Ting Lua, c*, Likun Pana* aShanghai

Key Laboratory of Magnetic Resonance, Sc0hool of Physics and Electronic

Science, East China Normal University, Shanghai 200062, China bKey

Laboratory of Spin Electron and Nanomaterials of Anhui Higher Education Insti

tutes, Suzhou University, Suzhou 234000, PR China cDepartment

of Chemical Engineering, School of Environmental and Chemical

Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China

*Corresponding author: Tel: +86 21 62234322; E-mail: [email protected] (Xianghui Wang); [email protected] (Ting Lu); [email protected] (Likun Pan)

1

Abstract Cobalt phosphides, as low cost and abundant non-noble materials for hydrogen evolution reaction (HER), are always constrained by their inferior charge transfer and sluggish intrinsic electrocatalytic kinetics. In this work, carbon-incorporated Co/Co2P microspheres (Co/Co2P@C) as a novel Mott–Schottky catalyst were synthesized successfully via carbonization and gradual phosphorization of Co based metal-organic frameworks. The unique merits, including Mott-Schottky effect at the interface formed between metal Co and semiconductor Co2P, the incorporated carbon-layer on the surface and the spherical structure endow Co/Co2P@C with favorable electrical conductivity, preferable kinetics and long-term stability when it was evaluated as electrocatalyst for HER in wide-pH range. As a result, the Co/Co2P@C with the optimized phosphorization degree delivers a benchmark current density of 10 mA cm-2 at the low overpotential of 192 and 158 mV in acidic and alkaline electrolytes, respectively, with a remarkable stability (CV cycling for 3000 cycles and continuous electrolysis at the overpotential of 200 mV for 48h). Therefore, the as-designed Co/Co2P@C should be one of the most promising catalysts for HER application. Keywords: Hydrogen evolution reaction; Metal-organic frameworks; Gradual phosphorization; Mott-Schottky catalyst; Wide-pH

2

1. Introduction Hydrogen has been deemed as the most potential carbon-free power source to alleviate energy crisis and decrease global warming [1-4]. Among numerous methods, hydrogen evolution reaction (HER) from electrochemical water-spitting is an economic and sustainable strategy to produce pure hydrogen efficiently [5, 6]. Except noble metals (e.g. Pt, Rh) and their oxides, numerous non-noble metal alternatives including chalcogenides [7-10], oxides [11-13], carbides [14-16] and nitrides [17-19] have been designed as state-of-the-art HER catalysts with desirable electrocatalytic activity so far. Despite tremendous efforts, there still exist few efficient and steady non-noble catalysts in both acid and alkaline electrolytes for the practical application of HER [20-23]. Thus, it’s imperative to explore promising earth-abundant noble-metal-free catalysts for HER with high electrocatalytic activity and excellent electrochemical stability in wide-pH operational condition [24]. Recently, transition metal phosphides (TMPs) have attracted much interest due to their excellent HER ability resulting from the more unsaturated coordination atoms on the surface of TMPs to capture more electrons [20]. Among various TMPs, cobalt phosphides have drawn

3

considerable attention owing to their relatively low cost, excellent redox property as well as high corrosion resistance in both acid and alkaline electrolytes [25, 26]. Especially, Co2P has widely been studied due to its satisfied electrochemical performance in photocatalytic hydrogen evolution [27, 28], supercapacitors [29], and lithium ion batteries [30, 31], and Co2P has also been evaluated as electrocatalyst for HER [32], oxygen evolution reaction [33] and overall water splitting [20]. However, cobalt phosphides always seriously suffer from the inferior electrical conductivity caused by their intrinsic semi-conductive property [34, 35], which limits their broad application in HER immensely. Currently, coupling semi-conductive materials with metal content to form Mott-Schottky structure is an effective strategy to promote the conductivity of semiconductors [36, 37]. The work function of metallic materials is often higher than the valence band of semiconductors and the electron can transfer through the interface between the metal and semiconductor [38]. Mott-Schottky effect at heterointerface can facilitate the

electron

transfer

efficiently.

Furthermore,

both

metal

and

semiconductor can cooperate synergistically as double catalyst for electrocatalytic reactions [39-42]. For example, nitrogen doped carbon implanted on nanowires arrays NC/CuCo/CuCoOx as an integrated Mott–Schottky electrocatalyst afforded an overpotential of 112 mV at 10

4

mA cm-2, which exhibited improved HER property than pure CuCoO [43]. Co/CoP bifunctional Mott–Schottky electrocatalyst exhibited a small overpotential of 253 mV at 10 mA cm−2 for overall water splitting, which was less than those of Co and CoP [36]. Metal-organic frameworks (MOFs) linked by metal ions and organic ligands are regarded as excellent precursors for energy storage and conversion

applications

including

Li/Na

ion

batteries

[44-47],

supercapacitors [48-50] and water splitting [51-53] due to adjustable structures and morphologies. Well-organized templating framework of MOFs can be converted to corresponding porous metal or metal oxide/carbon matrix after annealing under inert gas [54]. According to the previous researches, rational design of the catalyst structure with carbon supports can not only promote the conductivity, but also provide abundant active sites and enhance the stability in wide-pH electrolytes [55, 56]. Although MOFs-derived TMP Mott-Schottky catalysts are expected to display superior HER activities, related explorations have seldom reported by now. In this work, we demonstrate the design and fabrication of Co/Co2P@C microspheres as Mott-Schottky electrocatalyst by gradual phosphorization of carbon coated metallic Co microspheres derived from Co-MOF. A series of Co/Co2P@C microspheres with different ratios of

5

metallic Co content and Co2P content were synthesized. The obtained Co/Co2P@C composites are featured with excellent reaction kinetics, rapid electron transfer, long lifetime and rich electrochemical active sites resulting from the metel/semiconductor heterostructure, porous carbon shell and spherical structure. As expected, the Co/Co2P@C catalysts exhibit a current density of 10 mA cm-2 at the optimal overpotentials of 192 mV in acidic electrolyte and 158 mV in alkaline electrolyte, respectively, as well as desirable stability for 48 h in both aqueous solutions.

2. Experimental 2.1Chemicals Nafion 117 solution (5 wt%) was purchased from Sigma Aldrich. Co(NO3)·6H2O, benzene-1,3,5-tricarboxylic acid (BTC), NaH2PO2 ·H2O, ethylene glycol, ethanol, N, N-Dimethyl formamide (DMF), H2SO4 and KOH used in the synthesis process were of analytical grade from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China), and used without further purification. 2.2Synthesis of Co-MOF and Co@C microspheres The Co-MOF precursor was fabricated according to the reported solvothermal method [54]. In the typical process, 84 mg Co(NO3)2 ·6H2O 6

and 28 mg BTC were dissolved in a mixed solution with 20 mL ethylene glycol and 40 mL ethanol. The homogeneous pink solution was attained after continuous stirring for 3 h. Subsequently, the mixture was put into the 100 mL Teflon-lined stainless-steel autoclave and heated at 150 ℃ for 24 h. The Co-MOF was collected by centrifugation, washed with deionized water and ethanol for several times and dried at 60 ℃. At last, the obtained Co-MOF powder was annealed at 900 ℃ with a heating rate of 2 ℃ min-1 in nitrogen atmosphere for 1 h and cooled down naturally. The obtained black powder was Co@C microspheres. 2.2 Synthesis of Co/Co2P@C microspheres 50 mg as-prepared Co@C microspheres and a certain amount of NaH2PO2·H2O were loaded in the downstream side and the upstream side of a porcelain boat separately. The furnace was heated at 350 °C under N2 for 120 min with a heating rate of 2 °C min−1. The Co/Co2P@C microspheres

were

obtained

and

named

as

Co/Co2P@C-5,

Co/Co2P@C-10, Co/Co2P@C-15 according to the mass ratios of NaH2PO2 · H2O to Co@C microspheres (5:1, 10:1, 15:1), respectively. Furthermore,

the

Co2P@C

microspheres

were

synthesized

(NaH2PO2·H2O:Co@C=20:1) for comparison. 2.3 Material characterizations The surface morphologies and compositions of the samples were

7

characterized by field-emission scanning electron microscopy (FESEM, HITACHI, S-4800) and transmission electron microscopy (TEM, JEM 2010 JEOL). X-ray diffraction pattern was characterized by powder X-ray diffraction (XRD, Holland Panalytical PRO PW3040/60) equipped with Cu Ka radiation (λ=0.154 nm, V=30 kV, I=25 mA, 2θ=20o ~ 90o). The chemical information of samples was investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) equipped with a monochromatic Al Ka X-ray source. 2.4 Electrochemical measurements 10 mg catalyst powder was dispersed in 950 μL DMF and sonicated for 10 min. Then, 50 μL 5 wt% Nafion employed as the binder was added into the above mixture and sonicated for 10 min. Sequentially, a certain amount of the obtained catalyst suspension was transferred onto a glassy carbon electrode (GCE, diameter of 3 mm, ~0.07 cm2) polished previously and dried at the room temperature, yielding a mass loading of 0.2 mg cm-2. The modified GCE was also employed as working electrode for electrochemical measurement. In addition, a certain amount of catalyst ink was pasted onto a carbon cloth (~2 cm2, precleaned by sonication in diluted HCl, deionized water, and ethanol for 15 min in turn) and dried at the room temperature with a mass loading of 1 mg cm-2, which was employed for the stability test.

8

An

electrochemical

workstation

(Autolab

M204)

with

a

three-electrode system was employed to characterize the electrochemical properties, where a modified GCE, a Pt foil and a saturated Ag/AgCl electrode were applied as the working electrode, counter electrode and reference electrode, respectively. 0.5 M H2SO4 and 1 M KOH were used as electrolytes. In order to evaluate the potential versus the reversible hydrogen electrode (RHE), the potential against the saturated Ag/AgCl electrode was calibrated by the following equation: ERHE=EAg/AgCl + 0.1976 + 0.059*pH [57]. The linear sweep voltammetry (LSV) was carried out at a scan rate of 2 mV s-1, when the Tafel plots were calculated by LSV results. At the overpotential of 250 mV, the electrochemical impedance spectroscopy (EIS) was examined in the frequency region from 105 to 10-1 Hz with the amplitude of 5 mV [58]. Electrochemically active surface area (ECSA) was evaluated by the double-layer capacitance (Cdl), which was derived from a series of cyclic voltammetry (CV) tests at different scan rates in non-faradic region (0.10.2 V vs RHE) [59]. Mott-Schottky (M-S) analysis was performed in the potential region from -1 to 0 V vs RHE with the test frequency of 1000 Hz [60]. To investigate the stability of catalysts, the LSV measurement was conducted again after CV testing for 3000 cycles continuously in the non-Faradaic region [24]. Moreover, potentiostatic measurement at a

9

constant cathodic potential was performed in both aqueous solutions. According to the reported method [61], all electrochemical measurements were compensated for the iR drop in the tests (all data in this work were corrected if there is no specific expression).

3. Results and Discussion Fig. 1 illustrates the complete synthesis route of the Co/Co2P@C microspheres as Mott-Schottky electrocatalysts derived from Co-MOF by gradual phosphorization. Firstly, the Co-MOF was synthesized using cobalt nitrate and BTC (organic ligand) under solvothermal condition. Sequentially, the obtained Co-MOF was annealed to form Co@C microspheres under N2 atmosphere at high temperature while the organic ligand of MOF was decomposed. Then, after reacting with PH3 flow released by pyrolysis of NaH2PO2 · H2O, the metallic content in Co@C microspheres was gradually transformed into metal phosphide (Co2P) and the metal/semiconductor (Co/Co2P@C) heterostructure was successfully obtained. Meanwhile, the carbon shell transformed from organic ligand of Co-MOF was formed on the surface [54].

10

Fig. 1 Schematic of the synthesis route of the Co/Co2P@C microspheres.

FESEM images were used to investigate the surface structures and morphologies of the as-prepared samples. As shown in Fig. S1, the Co-MOF microspheres display smooth and regular spherical structure with a diameter range of 0.8 ~ 1.2 µm. After annealing at 900 ° C, the obtained carbon-incorporated Co microspheres derived from Co-MOF preserve the regular spherical structure (Fig. 2a). After reacting with PH3, the inner metallic Co was transformed into phosphide gradually while the sphere structure did not collapse. Co/Co2P@C-10 presents a microsphere structure assembled with small particles, as shown in Fig. 2b. Elemental mapping images (Fig. 2c) further confirm that the existence and well-dispersion of C, Co and P elements throughout Co/Co2P@C-10

11

microspheres. Meanwhile, the TEM characterization of Co/Co2P@C-10 in Fig. 2d further proves the uniform microsphere structure constructed by small particles and carbon shell with a mean diameter around 0.8 µm, which is in accordance with FESEM image. High-resolution transmission electron microscopy (HRTEM) image (Fig. 2e) shows two different interlaced lattice fringes with the interplanar spacings of 0.205 nm and 0.221 nm, which can be ascribed to (111) plane of metallic Co phase and (121) plane of semi-conductive Co2P phase, respectively. The existence of carbon shell (3 ~ 100 nm) on the surface of microspheres can be observed clearly from HRTEM in Fig. 2d and Fig. S2. The carbon layer implanted on the microsphere plays a vital role in facilitating the electrons transfer and enhancing the structural stability in catalytic process [55].

12

Fig. 2 (a) and (b) FESEM images of Co/C and Co/Co2P@C-10. (c) EDS images of Co/Co2P@C-10. (d) TEM and (e) HRTEM image of Co/Co2P@C-10.

Fig. 3a presents the XRD patterns of all as-prepared samples. Co@C displays three diffraction peaks at 44.2 ° , 51.5 ° and 75.8 ° , which are correlated with (111), (200), (220) planes of metallic Co (JCPDS No. 15-0806). The typical peaks of metallic Co can be still observed obviously in Co/Co2P@C samples after phosphorization, proving the existence of the metallic Co phase in Co/Co2P@C. The peaks at 40.7o, 43.3o and 48.7° in Co/Co2P@C can be indexed to (121), (211) and (031) planes of Co2P (JCPDS No. 32-0306) respectively, corresponding to the 13

HRTEM characterization (Fig. 2e). What’s more, diffraction peaks of Co phase in Co/Co2P@C samples become weaker with the increase of NaH2PO2 · H2O used in the phosphorization process while characteristic peaks of Co2P appear in Co/Co2P@C-5 firstly and exist stably in other Co/Co2P@C samples, indicating that the phosphorization degree can be efficiently controlled via changing the mass ratio of NaH2PO2 · H2O to Co@C. When the mass ratio of precursors was increased to 20:1, no characteristic peaks of metallic Co can be observed, indicating the complete conversion of Co@C into Co2P@C.

Fig. 3 (a) XRD patterns of Co@C, Co/Co2P@C and Co2P@C. (b) XPS survey spectrum, (c) Co 2p XPS spectrum and (d) P 2p XPS spectrum of 14

Co/Co2P@C-10. To investigate the chemical information of the as-obtained samples, XPS measurement of Co/Co2P@C-10 was conducted. The coexistence of Co, C, O and P elements is further confirmed by the survey spectrum in Fig. 3b. The high-resolution XPS spectra depict the elements Co and P in Co/Co2P@C-10, as shown in Fig. 3c and 3d. The Co 2p XPS spectrum can be fitted into two doublets, characteristic of Co 2p1/2 (Co3+ for 797.3 eV and Co2+ for 798.8 eV) and Co 2p3/2 (Co3+ for 781.7 eV and Co2+ for 783.2 eV), both of which shift slightly to higher binding energy [62]. Two typical peaks of metallic Co in Co/Co2P@C-10 are found at 778.6 eV for Co 2p3/2 and 793.6 eV for Co 2p1/2 [63]. In addition, the remaining two peaks centered at 785.7 eV and 802.1 eV are related to two satellite peaks of Co element [33]. In P 2p XPS spectrum (Fig. 3d), two peaks appearing at 129.6 and 130.2 eV are assigned to P 2p3/2 and P 2p1/2 in metal phosphide while a broad peak centering at 133.8 eV is attributed to the formation of P-O bond due to the surface oxidation of phosphide species [37, 64, 65], suggesting the successful phosphorization of carbon coated Co@C microspheres. The XPS result further illustrates the existence of metallic Co and Co2P in Co/Co2P@C-10. Electrocatalytic performances of the samples were investigated in N2-saturated 0.5 M H2SO4 electrolyte by a three-electrode system with iR 15

compensation. Fig. 4a exhibits the polarization curves of all catalysts. According to the previous research [66], the bare GCE does not exhibit HER activity. Compared with the pristine Co@C microspheres, significant improvement in HER performance has been found for Co/Co2P@C microspheres due to the strong synergistic effect between metallic Co and semi-conductive Co2P as double catalyst for HER. It is obvious that Co/Co2P@C-10 presents the optimal HER activity with an overpotential of 192 mV (ƞ10) to deliver a current density of 10 mA cm-2, while those of Co/Co2P@C-5, Co/Co2P@C-15 and Co2P@C are 220 mV, 208 mV and 212 mV, respectively. Furthermore, Co/Co2P@C-10 maintains superior electrocatalytic activity even at the higher applied potential. To explore the HER kinetics, the Tafel plots were obtained from the polarization curves. The Tafel slope values of all samples are listed in Fig. 4b and Table 1. All the Tafel slopes fall within the range of 38~118 mV dec-1, which suggests the Volmer-Heyrovsky process is the main reaction pathway for HER process [36]. According to the pioneering literatures [67-69], the negative charged P attracts protons via strong electrostatic affinity and transforms them into H intermediates, which accelerates the adsorption step (Volmer) dramatically. Generally, the reaction rate of Volmer step is much faster than that of Heyrovsky step in acidic media, 16

and thus the whole HER reaction is kinetically controlled by the Heyrovsky step [59]. It can be seen that the Tafel slopes of Co/Co2P@C decrease greatly compared with Co@C due to the formation of Co2P and Co/Co2P@C-10 exhibits the optimal Tafel slope. The enhanced HER kinetics of Co/Co2P@C can be attributed to the synergetic effect of metallic Co and semi-conductive Co2P. Co2P with more unsaturated coordination atoms on the surface can capture abundant electrons in the Volmer reaction and transfer electrons efficiently through the Mott-Schottky interface (as shown in Fig. 1). However, when the content of Co2P further increases, the samples show inferior kinetics, indicating that Mott-Schottky effect is affected by the ratio of metallic Co to semi-conductive

Co2P

in

the

17

electrocatalyst

[36].

Fig. 4 HER electrochemical activities in N2-saturated 0.5 M H2SO4 electrolyte: (a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist

plots

and

(d)

double-layer

capacitances

of

Co@C,

Co/Co2P@C-X and Co2P@C. Table 1 Electrocatalytic properties of Co@C, Co/Co2P@C-X and Co2P@C in acidic electrolyte.

η10/mV Tafel plot/mV dec-1 Rct /Ω Cdl /mF cm-2

Co@C

Co/Co2P@C5

Co/Co2P@C-1 0

Co/Co2P@C-15

Co2P@C

483

220

192

208

212

110.45

84.31

56.35

64.62

70.15

5.86

6.77

4.46

6.49

7.52

0.22

0.25

0.40

0.27

0.24

18

Electrical conductivity is regarded as a major factor that influence electrocatalyst performance of HER. The Nyquist plots are fitted with the equivalent electric circuit model as shown in Fig. 4c. All Nyquist plots include a semicircle in high frequency region related to the charge-transfer resistance (Rct) and an inclined line in low frequency region which reflects the ion diffusion process [70]. The fitted Rct values of Co@C, Co/Co2P@C-5, Co/Co2P@C-10, Co/Co2P@C-15 and Co2P@C are 5.86 Ω, 6.77 Ω, 4.46 Ω, 6.49 Ω and 7.52 Ω, respectively. Obviously, the smaller Rct reveals that the Mott-Schottky interface has played a significant role in facilitating charge transfer [24]. Furthermore, Co/Co2P@C-10 possesses the smallest Rct, indicating the expedited electron transfer and redox reaction at the interface, which is beneficial to the HER performance in acidic media. To further investigate the detailed reasons for the enhanced HER activity, ECSA was assessed by the value of the electrochemical double-layer capacitance (Cdl). CV test was conducted repetitively between 0.1 and 0.2 V versus RHE at different scan rates (Fig. S3). As displayed in Fig. 4d and Table 1, the Cdl of Co/Co2P@C-10 (0.40 mF cm-2) is greatly higher than those of other samples, demonstrating that Co/Co2P@C-10 microspheres possess the highest ECSA to offer more active sites for HER. The optimal Rct and

19

ECSA of Co/Co2P@C-10 can be attributed to the optimal mass radio of metallic Co and semi-conductive Co2P in Mott-Schottky electrocatalyst [55]. To evaluate the catalytic behavior of the samples in wide-pH environment, it’s necessary to test their HER activity in N2-saturated 1 M KOH electrolyte. Fig. 5a presents the iR-corrected polarization curves of all samples. Similar to the acidic media, all the Co/Co2P@C samples exhibit higher HER activity than pristine Co@C microspheres after the phosphorization treatment. Co/Co2P@C-10 microspheres achieve a current density of 10 mA cm-2 at an overpotential of 158 mV, better than of those of Co/Co2P@C-5 (207 mV), Co/Co2P@C-15 (193 mV) and Co2P@C (197 mV). More surprisingly, the ƞ10 values of all samples in alkaline environment is smaller than those in acidic environment, demonstrating the more excellent electrocatalytic activity in alkaline electrolyte. To analyze the electrocatalyst activity, fitted Tafel slopes (Fig. 5b), EIS (Fig. 5c) and ECSA (Fig. 5d and Fig. S4) were also measured and the results are listed in Table 2. It can be observed that Co/Co2P@C-10 exhibits excellent HER activity in alkaline electrolyte due to its promoted reaction kinetics, fast charge transfer and largest ECSA. Furthermore, the HER performance of Co/Co2P@C-10 as a Mott-Schottky electrocatalyst is better than those of most representative

20

non-precious electrocatalysts reported in the literatures under wide-pH environment (Table S1 and Table S2 in Supporting Information).

Fig. 5 HER electrochemical activities in N2-saturated 1 M KOH electrolyte: ((a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist

plots

and

(d)

double-layer

capacitances

of

Co@C,

Co/Co2P@C-X and Co2P.

Table 2 Electrocatalytic properties of Co@C, Co/Co2P@C-X and Co2P@C in alkaline electrolyte.

21

Co@C

Co/Co2P@C-5

Co/Co2P@C-10

Co/Co2P@C-15

Co2P@C

η10/mV

395

207

158

193

197

Tafel plot/ mV dec-1 Rct /Ω

176.18

109.13

64.42

85.55

97.13

6.18

5.22

3.82

4.94

7.28

Cdl /mF cm-2

0.28

0.31

0.47

0.43

0.38

Considering the semi-conductive property of Co2P, Mott-Schottky (M-S) analysis of Co2P@C and all Co/Co2P@C-X samples was performed to clarify the electrons transfer caused by formation of the integrated Mott-Schottky heterojunction. There exists a negative correlation between the slope in M-S plot and Nd [60], which indicates that the smaller slope represents the increasing charge carrier density. As shown in the Fig. 6a and Fig. 6b, the positive slopes confirm that the carrier is electron [71]. Meanwhile, the slopes of Co2P in M-S plots have decreased obviously after incorporating with metallic Co in both electrolytes and the slopes of Co/Co2P@C-10 reach minimum value representing Mott-Schottky

the

maximal

carrier

heterojunction

density,

accelerates

which the

verifies

electron

that

transfer

dramatically. The result of M-S measurement is consistent with the electrocatalytic analyses.

22

Fig. 6 Mott-Schottky plots of Co2P@C and Co/Co2P@C-X in (a) acidic electrolyte and (b) alkaline electrolyte. Moreover, electrocatalyst durability is one of the critical parameters for the practical application. Electrocatalytic stability of Co/Co2P@C-10 was examined by two methods, including accelerated degradation test and chronopotentiometry test in both two electrolytes (using modified CFP as the working electrode in two methods, catalyst loading: ~1 mg cm-2). CV cycles between 0.05 and 0.25 V versus RHE were conducted repetitively. After 3000 CV cycles, the polarization curve was measured again in the same condition and it can be found that the Co/Co2P@C-10 microspheres can still perform efficient HER activity (Inset images in Fig. 7a and Fig. 7b). The η10 of Co/Co2P@C-10 in acidic and alkaline electrolytes only increased 15 and 35 mV, respectively. Moreover, potentiostatic electrolysis measurement was also repeated at a constant overpotential of 200 mV to assess the stability for three times. With continuous evolution of robust hydrogen gas, the current density had been maintained for 48 h 23

with little decay as shown in Fig. 7a and Fig. 7b. The current density remains 83.26% and 82.13% of original values in acidic and alkaline electrolytes, respectively (Fig. S5). The FESEM and TEM images of the post-HER Co/Co2P@C-10 after potentiostatic measurement are presented in Fig. 7c, Fig. 7d and Fig. 7e, confirming that the microsphere structure of Co/Co2P@C-10 does not collapse after the long-term stability test. In Fig. 7f, two different interlaced lattice fringes with the interplanar spacings of 0.205 nm and 0.221 nm ascribed to (111) plane of metallic Co phase and (121) plane of semi-conductive Co2P phase still can be observed clearly. Mott-Schottky heterojunction does not disappear after the long-term electrolysis test. All observation indicates the satisfied electrocatalyst durability in wide-pH environment which benefits from rapid electron transfer caused by Mott-Schottky interface, porous carbon shell supporting and spherical structure.

24

Fig. 7 Evaluation of HER catalytic durability for Co/Co2P@C-10 in both electrolytes: potentiostatic curves of Co/Co2P@C-10 in (a) acidic electrolyte and (b) alkaline electrolyte (Insert: LSV curves before and after 3000 CV cycles). FESEM images (c, d), TEM image (e) and HRTEM image (f) of Co/Co2P@C-10 after the potentiostatic test.

4. Conclusions

25

In summary, Co/Co2P@C microspheres used as Mott-Schottky electrocatalysts

were

fabricated

from

Co-MOF

precursor

via

solvothermal reaction with subsequent carbonization and gradual phosphorization process. The optimized Co/Co2P@C-10 microspheres exhibit excellent HER performance in wide-pH operational condition, especially in alkaline electrolyte (10 mA cm-2 at 158 mV), the overpotential of which is much less than reported works, such as CoP (10 mA cm-2 at 209 mV) [57] and CoOx (10 mA cm-2 at 232 mV) [72]. The favorable

catalytic

property

of

Co/Co2P@C

Mott-Schottky

electrocatalysts is ascribed to the following factors (i) high electrical conductivity benefiting from Mott-Schottky effect at the heterointerface, (ii) synergy effect of metallic Co and semiconductor Co2P as double electrocatalyst and (iii) porous carbon shell. Although the HER activity of Co/Co2P@C is improved, the overpotential is still high for the practical application, which should be further investigated in the future. The strategy in this work will offer a novel method to explore metal/semiconductor hybrids as Mott–Schottky electrocatalysts. Acknowledgements This work is financially supported by the Provincial Natural Science Foundation Development

of

Anhui Program

(1908085ME120), of

Anhui

26

Primary

Province

Research

and

(201904a05020087),

Innovative Research Team of Anhui Provincial Education Department (2016SCXPTTD), and Key Discipline of Material Science and Engineering of Suzhou University (2017XJZDXK3).

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Graphical Contents Entry

Co/Co2P@C Mott-Schottky electrocatalysts were successfully synthesized from Co-MOF and show excellent HER activity owing to the Mott-Schottky effect, synergy effect of Co and Co2P and porous carbon shell.

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CRediT author statement

Huangze Yu: Methodology, Investigation, Writing-Original Draft. Junfeng Li: Visualization, Validation. Guoliang Gao: Data Curation. Guang Zhu: Resources. Xianghui Wang: Formal analysis, Project administration. Ting Lu: Conceptualization, Writing-Review & Editing. Likun Pan: Supervision, Writing-Review & Editing.

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Declaration of interests

☒

The authors declare that they have no known competing financial interests or

personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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